Photoacoustics is a hybrid imaging technique that combines the contrast available to optical imaging with
the resolution that is possessed by ultrasound imaging. The technique is based on generating ultrasound from
absorbing structures in tissue using pulsed light. In photoacoustic (PA) computerized tomography (CT) imaging,
reconstruction of the optical absorption in a subject, is performed for example by filtered backprojection. The
backprojection is performed along circular paths in image space instead of along straight lines as in X-ray CT
imaging. To achieve this, the speed-of-sound through the subject is usually assumed constant. An unsuitable
speed-of-sound can degrade resolution and contrast. We discuss here a method of actually measuring the speedof-
sound distribution using ultrasound transmission through the subject under photoacoustic investigation. This
is achieved in a simple approach that does not require any additional ultrasound transmitter. The method uses
a passive element (carbon fiber) that is placed in the imager in the path of the illumination which generates
ultrasound by the photoacoustic effect and behaves as an ultrasound source. Measuring the time-of-flight of this
ultrasound transient by the same detector used for conventional photoacoustics, allows a speed-of-sound image
to be reconstructed. This concept is validated on phantoms.